Phototube and method of manufacture



vJuly 16, 1946.A

J. E. HENDERSON ETAL PHoToTUBE's AND METHOD 0F MANUFACTUBE Filed Oct. 14, 1945 lNvl-:NoRs JosEPH EHENDsnsoN B ROBERT BJQNES W ATTORNEY Patented July 16, 1946 Joseph E. Henderson, Seattle, Wash., and Robert B. Janes, Verona, N. J.; saifd Henderson assignor to the United States of America, as represented by the Secretary of Navy and said .lanes assigner to Radio Corporation of America, a corporation of Delaware l Application October 14, 1943, Serial No. 506,274

- 1 Our invention relates to photo-electric tubes and their-method of manufacture, and particularly to tubes incorporating dual cathodes having coatings of elements such as antimony, arsenic andbismuth sensitized with an alkali metal.

In co-applicant Janes copending application, Serial No. 434,681 filed March 14, 1942, there was described a phototube of a very rugged type capable of withdrawing high values of acceleration both longitudinally of the tube and when the tube is subjected to an'angular acceleration, or centripetal acceleration such' as exists when a whirling motion aboutits'longitudinal axis is'given to the tube. In this prior tube it was substantially impossible to obtain a very close spacing between the anode and cathode While still retaining its rugged character and'v high sensitivity. For example, co-applicant .Janes found that it was necessary to separate the anode and cathode by a distance of at least three-quarters ofan inch. Certain applications require closer spacing than this and, furthermore, such lapplications require the use of a second cathode which preferably serves as an anode to the rst cathode,

It is an object of our invention to provide a very rugged phototube of the dual cathode antimony, arsenic or bismuth coated type; It is another object toprovide a dual cathode phototube of the alkali metal treated antimony, arsenic or bismuth type, wherein the cathodes may be placed very close together. It is a further object to provide .a phototube of the dual cathode type which will withstand high forces of acceleration without distortion or mechanical failure of the parts or variation in the operating characteristics of the tube while providing a structure which may be easily processed .and having high photosensitivity. It is a further object to provide an improved method of manufacturing a very rugged phototube o-f the dual cathode antimony, arsenic and bismuth coated types; It is a still further obj ect Ito provide a method of manufacture allowing the production of dual antimony coated cathodes withoutdeterioration of the antimo-ny coatt ing or coatings during processing.' l

yThe foregoing and'other objectsyfeatures and advantages of our invention will 'be apparent when considered in viewof the following description and the accompanying drawing wherein,

Figure 1 is a View in longitudinal section of a Claims. (Cl. Z50- 465) 2 Figure 4 is a longitudinal sectional View of apparatus suitable for performing one of the principal operations involved in the manufacture of the phototube hereinafter described.

In thel following description we will refer specically to antimony as a cathode coating and it will be appreciated that arsenic and bismuth are the full equivalents of antimonyy and may be substituted in whole orin part for the antimony without departing from the scope ofour invention. f V' i In accordance with our invention we have found that a tube comprising a tubular section or sections of insulating material and closed at opposite ends by metal closure caps may be assemb'led with an additional cathode' intermediate the closure caps if certain precautions are taken in the order of assembly and in the application ofthe antimony coatings on the cathode foundations, such as on one of the closure caps and the l intermediatecathode foundation. More particularly we have found that a tube of the type describedin co-applicant Janes above-mentioned application may be constructed with an additional photo-cathode intermediate opposltely disposed envelope closure caps serving as anode and cathode respectively with the two cathodes exceedingly close together, provided the cathode foundations are provided with their antimony coatings following the sealing of the foundations or foundation supports inthe tubular envelope.

Referring to Figure 1 whereinv we have shown ka phototube having dual cathodes made in accordance with our invention, the tube comprises two end closure caps land 2, each of which is sealed to Vpre.determined lengths o f glass tubing 3 and 3e. The minimum axial length of the tub-- ing 3 is not critical andmay be considerably less than AVone-'half inch although the minimumlength of .thevtubinggga must not `be less than'one-half inch Vandis preferably not less than three-quartersof an inch. Preferably one of the caps; such as' "rl-he#A cap I, is aperturedas shown at 4 and is phototube made in accordancewith our invention,

Figure 2 is a cross sectional view of the phototube shown in Figure 1 taken along the lines 2-2, 'i

provided with aglassftubulation 5 sealed around the'aperture `f or evacuation and sothat following evacuation the tubulat'ion' maybe closed Vsuchwa's by a Vtippirig-off operation. The opposite closure cap 2 Aisnon-aperturedand supprtsasource 6 ofV 'alkali metal such as a caesium compound rev'ducible toY caesium upon vthe application of hea/t to the central outer area of the capl.

In addition 4and in accordance with our -inven- 4tionwe provide intermediate the closure caps vI and 2 `a secondcathode foundation] at a distance, i

:preferably desertores .Closure cap l 'than thhlthe general direction of the cap 2 but shieldedA therefrom by the second cathode foundationl. As shown in Figure 1, thereasons for which ap-L pear more fully hereinafter, the secondcathode foundation 'I is supportedat aV relatively'short distance from the end cap'I land at a larger disthe tubing 3 and 3a. A chrome-iron which we have found satisfactory for such use with lime glass contains as the principal constituents-k Percent Chromium 26-30 Nickel 0.5 Manganese 0.5 Silicon 0.5 Iron Balance tance from the end cap 2 -by vsupporting means t such as a ring-shaped or annular support member 8. It has been found that the foundation'l cannot be photo-electrically sensitized with an alkali metal treated antimony, arsenic or bis` muth coating if the distance between the end cap 2 and the coating is less than a predetermined minimum, such as less than one-half inch although this distance is preferably not less than threequarters of an inch. y However, in accordance with our invention/We are able to provide very close spacings ,less'than the minimum distance-,of onehalf inch by following oury improved methodof. manufacture. Since the cathode foundation 'I serves as a cathode with respect to thefcap-and'as an anode withrespect to the end cap I ,A andlthe end cap I is made'photo-sensitive, the relatively close spacingV betweenthe foundation'land end cap I provides characteristicswhich were unobtainable in tubes of the types described using antimony coatings.l Consequently, in accordance with our invention wek provide,following the assembly of the end capl coating I2 o f an element such as antimony either 'ri'pgiby the maar tabs, l l.

-More particularly we have found Ythat it is not beforeorafter its attachment to the support lonlylydesirable but necessary to provide the antimony coatings 9 ,and I2 at a definite time during'rthe sequential manufacturing steps to prevent Vinjury to these coatings during subsequent steps of themanufacturing processes. For eX- ample, .if thel coatings 9y and I2 are subjected to relatively high temperatures, such as occur when the closure cap I is sealed to the glass tubing 3 orwhen the support rings is sealed between the tubularglass portions 3 and 3d, permanent injury occursmto the coatings. The sealing operations f musthecessarily Vbe performed in the presence ofnorrnal atmosphericconditions *and our sejquence ogprocessingsteps avoids or substantially l tesV material oxidationof the antimony cpatir,lgsl on the foundations. g Furthermore, the

,closure cap I and the support ring 8 must be subje edtoce rtainprocessing steps to provide good glproperti'es at the time the seals aremade `.hetwee,nrlthesemembers and the glass tubings 3 Vand 3d, suchstepsfbeing incompatible with previouslyapplied antimony coatings.

Y ,The `closure caps ;I- and 2, as well as the ring mber, arepreferablyof material such as chrome-iron having a, cov-efficient ofexpansion:A

Following formation of a closure cap I to a satisfactory form, onecap for each tube is apertured as shown at 4 andwhere a foundation member other than the cap for the first cathode is desired, such as the disc IB shown in Figure 3, the disc vII?) is attached to the cap I, such as by welding. The

material of the disc I0 as well as of the founda-f tion 1 is preferably of high chromium content alloy such as chrome-nickel, the content of chromium ranging from 5 to 30% by weight, the alloy being oxidized to a greenish color as disclosed in Janes" copending application, vSerial No. 481,246 filed March 31, 1943. Thetubulation 5 is then sealed around the aperture '4 and the cap I oxidized to provide'a film of oxide which forms a good bond` between the'cap I and thev glass tubing 3 during the next operation' of sealing these two parts together. Such oxidation may occursimultaneo'usly with thesealing of -the tubulation 5 to the cap I',f.although alternatively'the formed caps may beuoxidizedV in moist vhydrogenV as described byHull, vBurger and Navis in anarticle entitled, Glassfto-metal seals, part 2, "J curnal of lApplied Physics, volume 12, No; 9,' September 1941', pages 698-707. Such iiring produces a relatively thick coating of greenish'colored chromic oxide integral with the surface of the capV land the disc I0, if such vdisc isused as 'the cathode foundation. If the moist hydrogen' oxidation step is used we prefer to bake the end caps for a sufficient length oftime," such as in a non-oxidizing atmosphere at atemperatureof 500 C. forabout twelve hours to removeany occludedhydrogen. 'l'he seal betweenthe end capV IV and the glass tubing 3^may be. made'byrotatin'g the parts 'in the presence of Vfires heating" the rim of the cap and the lower end of the tubing 3. 'vWe use a similar sealing step to attach the support ring 8 to the upper end of .the glasstubing 3 and to the' lower en d ofthe glass tubing'Sa.' All 'of these steps are performed 1n accordance with our invention before the applicationof theantimonycoatings S and' I2, the cap I'and the foundation I only then being ready for the application ofthe antimony coatings 9 and Il which are applied in separate steps; f

Referring toFigure 4 and prior toattachinent of the foundation 'I tothe ring member 8,' we insert the glass tubing 3"and a'ove'r a tubular or cylindricalopen endedlshieldV I5, theshield extending'ltowithin close proximity of the endcap nI' or the disc I 0,' if such disc is "used, anden-which the' antimonyf vis to loe-deposited. l The .shield AI5 4is preferably.supportedrwithin a'chamber or bell jar lcapable'of being '.evacuated."y We"alsofsup" Vport, opposite the'cap I a? source ofantimcny to be vaprizle'd and condensed -'thereonJ Such a source ofanti'inony may comprisea refractory metal; 9011y l1 supportingy a quantity 4Vof 'antimgny I8 onotherelement such/asV arsenic or,` .bismuth and whichvmay belvaporizedrby passing an elec- Ktric current through the. connecting leads i9. 4,lollotv'ing therinrsertion of thOpen endeV Lof- 'theV tubeshown in Figurell -the bell` `J djvv'aiis I ar'II is evacuatedftoaghigh .Vacuums A.the .residuallair A`substantially matched with that ofthe glass of 7 pressure preferablymbein less than 40.5 "microns thickness `of this antimony coating Amay Arange fromva thickness'corresponding to 0.1 to 0.'16mi1 ligram per square centimeterywhichis sufficient to provide good sensitivity when such a coating is treated with alkali metal. The rate of vaporization and condensation is preferably'low, being less than 0.2 milligram per'square centimeter of the surface to be vcoated per second, since higher rates of condensation may beA detrimental, forming a non-uniformcoating. The entire :bell jar assembly With the exception of the coil I1 and the heated antimony source I8 is preferably maintained at room temperature during the evaporationand condensation steps, except for `such slight heating of the cap I as may occur by radiation fromthe The Yantimony coating I2 on the ,foundation 1 may be applied to a similar thickness in a similar manner either in the bell jar I6 following Aattachment of the foundation 1 to the yring member 8 by the metal strips I I, in which event'the shield I5 is made somewhat'shorte'r terminating 'adjacent the upper surface of the foundation 1,l `al though the antimony coating I2 may be applied prior to its attachment while separately supported in vacuum with-exposure to a heated antimony source. If this latter method is used the foundation 1 is affixed to the ring member 8 following the application of the antimony coating I2.

I n accordance with our invention and following theapplication'andiassembly steps referred to above, the envelope formed by the tubular glass sections 3 and 3a. is closed by sealingjthe vcap 2 to the upper end of the tubing Salat a sufficient distance, such as three-quarters of an inchv from the surface of the nearest adjacent antimony coatiing, such as the coating I2. A source 6 of alkali metal such 'as a caesium bearing compound jis first valii-ized to the inner surface of the end cap 2, such as by enclosing al compound of caesium in a'small apertured container, whereupon'the cap 2, previously oxidized as described above, is sealed to the upper end of the glass tubing 3a.. To pre-k vent'excessive heating of the antimony coatings during this nal sealing step, we prefer toV cool the ring member 8 as well as the yend cap I, such vas by a blast of air directed thereon during the sealing -of the cap 2 to the upper end of the glass The'only other,u precaution which need be followed in the assembly of `our'dual cathode phototube is the maintenance of the antimony coatings in the absence of high concentration of water vapor, especially during the time these coatings are subjected to temperatures above normal room temperature. Consequently the relative humidity of the atmosphere to which the coated cathode foundations are subjected should be low and preferably less than 80% relative humidity at 25 C., which corresponds to a water vapor-concentration equivalent to 8 grains of water vapor per cubic footvof air. The satisfactory results obtained are believed to be due to the fact that antimony does not oxidize when subjected to normal atmospheric conditions in the presence of low humidity.

Following the assembly of the tube as shown in Figure 1, the internal Volume thereof is exhausted through the tubulation 5 and the tube is baked at a temperature of 275 to 310 C. to remove any occluded gases contained in the tube parts. Following this baking which may be from `ifteen 6 minutes to one-half hour, we vapor-ize alquantitv of alkali metal such as caesium from thesource 6 to sensitize both of the coatings -9 and I2-. To allow -access of the alkali metal to the antimony coating 9 the foundation 1 is provided with 'an aperture 4a although sufficient spacing `may be provided about the periphery of the foundation.

Following the vaporization of the alkali `metal the tube is baked, preferably at a temperature `of C., for thirty minutes and during this baking the alkali :metal apparently combines with Athe antimony'coatings to provide photocathode surfaces having very high sensitivity to light. Following the baking the tubulation 5 may be tipped-off and the tube removed from thel pump and preferably rebaked at a'temperature of 160 C. fora period of twenty minutes, following which'the tubeis` aged to further stabilize the photoelectric emission from the cathode.

In operation the cap I serves as a cathode connection and is operated at a negative potentialV with respect to the ring member 8 which supports and is electrically connected to the cathode foundation 1, this member likewise being maintained at a negative potential with respect to the .anode cap 2. The'electrons liberated from the coating 9 are ldirectly proportional to thelight incident thereon through the glass tubing 3 and similarly electrons liberated from the coating I-2 and collected by the anode cap 2 are directly proportional to the light incident on the coating I2 through theglass tubing 3a. Consequently the disclosed structure isv ideally adapted to -inulti-V directional use wherein either one or more light sources. or a broad source such as the sky are fusedv since the' cathode'smay be illuminated from all azimuths' on sides facing theA respective fanodes, I i

While we have `described AourA invention with particular reference to a dual-cathode phototube Vand'. have set forth our,A preferred method of manufacturing such a tube, it will be appreciated that various modiiications'such as multiple cathode phototubeacan be made both in the structure fandgin the method of manufacturing without :departing from the spirit of our invention orthe scope Vthereof as set forth in the appended claims# l' We claim: l 1f. A phototube comprising a tubular glass envelope, a metal closure at each end of and forming the endwalls of said envelope, -al cathode foundation extending transversely of said envelope intermediate said closures, a coating selected from the group consisting of antimony, arsenic, and bismuth on the surface of said foundation facing one of said metal closures, a similar coating on the other of said closures and alkali metal within said envelope to sensitize said coatings.

2. A phototube comprising a tubular glass envelope, a metal cap sealed to each end of said envelope, a coating selected from the group consisting of antimony, arsenic, and bismuth on one of said metal caps as a cathode foundation, a second cathode foundation extending transversely of said envelope between said caps, a coating selected from the group consisting of antimony, arsenic, and bismuth on the surface of said foundation facing the other of Said caps, and a quantity of caesium within said envelope to sensitize said coatings.

3. yA phototube as claimed in claim 2 wherein said coatings are of antimony.

4. A phototube comprising a tubular envelope Y .7 of -transparent material, `a metal cap. sealed .to and'closing'each ofthe opposite ends of saidvenvelope, a cathode foundation extendingl transversely of said -envelope between saidmetalcaps and being spaced more closely to one of said caps than the other cap, a coating selected fromrthe group of elements consisting of` antimony; arsenic andgbismuth n said one of-said4 caps andga secondi Coating selected `from Asaid group of elements on,` tlfiezsurface Vof said foundation facing the other ofi-said caps.;` and anv alkali-metal deposited oneach o fsaidcoatings' to render said coatings photo=emissive. v Y I -Y 5, 1A: phototube-.comprising a Vcylindricalv glass envelope, af metal cap at each'end of said envelope,. ;one' of said caps being lapertured and closed by an lexhaust tubuiation, la-'cathode foundationafxed-directly to oneY of; saidcaps 'and facing in the direction of the other-of said caps,

,ak second cathode foundation Vextending transversely across said envelope parallel to and between said caps a-nd closer to said iirst cathode foundation than to the other end cap, and a coating of alkali metal treated antimony on each of said, cathode foundations. Y

6. A phototube comprising a pair of coaxial, separated transparent envelope portions sealed at their adjacent ends to a metalsupport ring in a plane transverse to said envelope, a metal cap sealedto each of the opposite ends of said envelope portions, a coating of alkali metal treatedantimony onrone of said metal caps and a photosensitive electrode extending transversely across said envelope and supported by said metal supuport ring between said coating and the other of said metal caps and having a photo-sensitive surface facing said other metal caps.

7. A phototube comprising a. pair of longitudinally spaced coaxial transparent envelope portions sealed at their adjacent ends to an electrode support member, one of said portions having anaxial length less than three-quarters of an inch, the other having a minimum length of approximately one-half to three-quarters of an inch, a Dhotocathode attached to and carried by said electrode support, a metal end cap closing the open end of each of said envelope portions and a coating of caesiated antimony on the cap-sealed to that envelope portion whose minimum length is approximately one-half tov three-quarters, of aninch. H

8. The method of manufacturing a phototube having two closure caps separated by a tubular insulating member and an intermediate electrode comprising sealing one or said caps toene end of said member, sealingafsuppo'rt'fora photosensitive electrode intermediate the .ends of `said member, depositing an element selected fromxthe group consisting of antimony,.arsenic and bismath-on a-surface in'electrical contact with the cap 'sealed to oneend of said'menber, attaching an electrode foundation `'adapted to bel photoelectricallysensitized Vto said support and then sealing theother cap to the other end of said member, evacuating the envelope formed by said member and said.A caps, land sensiti'zing said deposited element and said foundation with an alkali metal.

9. vThey method of manufacturing a Vphototube having an envelope of two metal closure caps sealed to and separated by sections of glass .tubing comprising sealing one of said closurefcaps to [one end ofv one of said sections of glass tubing, sealing a support for a photosensitive electrode tothe opposite-end of said one-section,seali ng one end of ther-other section of glass ltubing to said support coaxial with said one section, deposit- ,Y

ing .a coating of an element which when sensitized with an alkali metal is photo-sensitive on a surface vadjacent said one closure cap, attaching a foundation which when sensitized with an alkali..metal Vis photo-sensitive or'said support, sealing' the other closure cap to the other end of said-other glass tubing section to form an envelope,fevacuating said envelope and sensitizing said coating and said foundation with an alkali metal.- Y i' 10`. The method Vof 1 manufacturing a phototube having two closure caps separated by tubular y insulating material comprising sealing one of said .closure Vcaps to one end of said tubular insulating material, sealing an 'electrode support to said insulating material intermediate the opposite ends thereof, inserting an open-ended shield with-l in the Aopen end of said tubular insulating material beyondsaid electrode support and to within close proximity of said sealed'closure cap, evaporating and-condensingantimony on a surfaceadjacent saidclosure cap while the inner walls of said insulating material and said electrode support are vprotected from condensation rthereon by said open ended shield, removing said shield, attaching an antimonycoated electrode to said-electrode supi port, sealingthe other closure cap to the open end `of said tubularinsulatingmaterial to'form an envelope evacuating said envelopeand sensitizing said antimony with an alkali metal.

JOSEPH E, HENDERSON. ROBERT B.' JANE'S. 

